It should have been negative since it is returned with ERR_PTR().
Introduced in new code commit:
   commit 
50470bb011c4be278097670bea92462f4e8c8945
   Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
   Date:   Mon Mar 23 11:10:36 2015 +0000
    drm/i915/skl: Support secondary (rotated) frame buffer mapping
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
 
        struct sg_table *st;
        unsigned int tile_pitch, tile_height;
        unsigned int width_pages, height_pages;
-       int ret = ENOMEM;
+       int ret = -ENOMEM;
 
        pages = obj->base.size / PAGE_SIZE;